How to cut a .obj file in half?
You can use the Bisect Tool for that.
It's pretty simple.


In Edit Mode, select the whole mesh through Select > All
Use the Bisect Tool and adjust the parameters to your needs
Enable X-Ray Mode and select the parts you want to delete


Note: If you don't need a sharp cut, step 3. should be enough :)



Use the Bisect Tool to cut the object in half



Then enable X-Ray mode and select the parts you want to delete